Among the advantages offered by digital oscilloscopes is the ability to display the same captured data in different ways. The ability to do this arises from there being more captured data than is displayed at any one time on the screen. Depending upon certain constraints connected with sampling rate, memory size and selected sweep speed for the displayed waveform image, one would like to be able to have the oscilloscope reformulate the displayed waveform image to effect zooming in or out (changes in sweep speed) or panning (what segment of the waveform is displayed relative to the trigger). High performance top-of-the-line digital oscilloscopes allow these sorts of operations, but at the price of possessing an expensive architecture. They have very fast sampling rates and relative large sample memories organized as one long time axis. These may be termed "real time" oscilloscopes. Even so equipped, there is often a significant sluggishness in responsiveness to the controls of a real time scope when reformulating the displayed waveform image, owing to the techniques used to interpolate pixels in the frame buffer for x axis locations in the frame buffer not having a corresponding y value in a data record stored in the sample memory.
An architecture that can be used in lower cost digital oscilloscopes of modest performance is one called "random repetitive sampling". The February 1992 issue of the Hewlett-Packard Journal is devoted almost entirely to a description of an early form of this digital oscilloscope architecture (in the form of the then new products HP 54600A and HP 54601A oscilloscopes). In random repetitive sampling the signal to be measured is sampled at a fairly low rate for perhaps a considerable length of time; or a time at least long enough to acquire one sample per pixel position in the x direction (ala real time). This produces a single instance of an acquisition record. This single acquisition record would be suitable for forming a displayed trace if the time span of the displayed trace were to equal the time interval expended to acquire it, and it contained a sufficient number of samples. If that were all there were to random repetitive sampling, then it would be nothing more than very low performance real time sampling. Instead, steps are taken to automatically arrange that the segment of the waveform to appear on the screen is but a fraction of the acquisition record. It will be understood that the points in that fraction are too far apart to produce a satisfactory display. However, owing to the repetitive nature of the waveform, it is possible to acquire many consecutive acquisition records. The performance of the architecture becomes quite acceptable when steps are taken to take notice of the particular offset in the time axis of each acquisition record relative to its associated trigger event, and to ensure relative randomness (as between acquisition records) of the start of the time axis, so as to avoid both redundant samples and holes where there are no acquired data for use in the intended display. With this arrangement the display is formed by investigating the data in not just a particular segment of a single acquisition record, but for an entire collection of such records. If the collection of acquisition records is large enough, there will be almost no holes. It allows pan and zoom by simply selecting the segment of interest in the collection of acquisition records. Panning is accomplished by sliding the segment of interest back and forth along the time axis of the acquisition records, while zooming changes the width of the segment of interest, and relies on there being many acquisition records whose time axes are aligned so as to avoid holes and redundant samples. This scheme does not have the single shot bandwidth that real time does, but is a good compromise for viewing repetitive signals.
Typically, the inspected segments of the acquisition records contribute their data to a waveform record, which when complete, is loaded into a frame buffer composed of video RAMS's. It is the contents of the frame buffer that actually appear on the screen. Now, consider the process of inspecting the acquisition record segments to build the waveform record. There may be, perhaps, one hundred acquisition records, of which only ten percent of each record is the segment presently of interest. Let us say, for the sake of explanation, that each acquisition record holds data for five microseconds worth of time. Each acquisition record is made subsequent to a trigger, and will hold data values for, say, one thousand consecutive samples, say, five nanoseconds apart. The triggering event might not be so agreeable as to occur on exact multiples of the 5 ns sample clock; it can happen whenever it wants to. This gives rise to the trigger offset mentioned above. To characterize the offset it is sufficient to note the time interval between the trigger event and a nearest (either always before or always after) sample clock. The period of the sample clock sets a definite maximum value for the trigger offset. In a conventional digital oscilloscope the newest acquired data is simply stored in the next available acquisition record.
If the signal being measured is ideally repetitive, and never changes, then all the data in the acquisition records is equally valid, and no anomalies will occur in the construction of the waveform record. It can happen, however, that certain acquisition records are "outdated", in the sense that they were made a relatively long time before the latest one. It can be the case that the data in an older acquisition record is, while not wrong, in significant disagreement with newer data, and ought to be suppressed. This happens naturally in a (non-storage tube) analog oscilloscope, owing to the decay of the trace (finite, and short, persistence). It would be desirable if a random repetitive oscilloscope could avoid displaying data older than is appropriate for the measurement being made.
It may also happen with a conventional digital oscilloscope that there is not a random distribution of the trigger offset values represented in the acquisition memory. While this condition cannot entirely be avoided, simple sequential storage of acquisition records in the order that they occur can compound the problem by using up the acquisition memory during an accumulation of too many acquisition records of similarly valued of trigger offset. It would be desirable if a random repetitive digital oscilloscope wold operate in a way that maximized the retention of a collection of acquisition records having a random distribution of trigger offset vales.